Re: [Xen-devel] Re: poor domU VBD performance.

[Steven Hand <Steven.Hand@xxxxxxxxxxxx>]

> On Tuesday 29 March 2005 02:13, Ian Pratt wrote:
> > > It looks like there might be a problem were we are not
> > > getting a timely
> > > response back from dom0 VBD driver that the io request is
> > > complete, which
> > > limits the number of outstanding requests to a level which
> > > cannot keep the
> > > disk utilized well.  If you drive enough IO outstanding
> > > requests (which can
> > > be done with either o-direct with large request or a much
> > > larger readahead
> > > setting with buffered IO), it's not an issue.
> >
> > Andrew, please could you try this with a 2.4 dom0, 2.6 domU.
> 
> 2.4 might be a little while for me, as I an running Fedora core3 with udev.  
> If anyone has any easy way to get around the hotplug/udev stuff, then I can 
> do this.

You can run a populated /dev "underneath" the udev stuff quite happily; 
e.g. if you boot into FC3 w/ udev do: 

  cd /dev/ 
  tar zcpf /root/foo.tgz . 

If you can boot from a rescue CD or sim, just mount your FC3 
partition and untar the device nodes.

Works just fine. 


> I did run a sequential read on a single disk again (using noop IO schedulers 
> in both domains) with various request sizes with o_direct while capturing 
> iostsat output.  The results are interesting.  I have included the data in a 
> file because it would just line wrap an be unreadable in this email text.  
> Notice the service commit times for domU tests.  It's like the IO request 
> queue is being plugged for a minimum of 10ms in dom0.  Merges happening for 
> >4K requests in dom0 (while hosting domU's IO) seem to support this.

[snip]
 

Ah - thanks for this -- will take a detailed look shortly. 

cheers,

S.

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